A number of materials are available for fabricating structures, coatings and liners for use in, particularly, corrosive applications. Among these materials are resins such as polyvinyl chloride, polypropylene, and chlorinated polyvinyl chloride, all available commercially in a large variety of grades. These resins, while providing generally satisfactory resistance to many acids and organics, can be susceptible to attack where even small quantities of certain organics such as benzene or chorinated benzenes are present. Other resins, such as Teflon.RTM., a product of E. I. duPont available as either hexafluoropropylene copolymer (FEP) or tetrafluoroethylene (TFE), and Kynar.RTM., polyvinylidene fluoride, available from Pennwalt have been used for structures, liners and coatings but, upon occasion, have experienced difficulty. For example, TEFLON coatings are somewhat porous, requiring a substantial thickness to protect an underlying substrate reliably. KYNAR, in some applications, has show swelling and resultant delamination when used for coating.
Rubber and synthetic rubbers such as Hypalon.RTM. and Viton-A.RTM., E. I. duPont products, have been used for corrosion resistant structures, coatings and liners but remain subject to swelling and delamination in the presence of certain corrosives and in the presence of even trace quantities of certain organics.
One group of contemporary copolymers offering potentially gratifying advantages when used for forming corrosion resistant structures, coatings and linings are the fluorocarbon vinyl ether copolymers having pendant functional groups based upon SO.sub.2 halide and CO halide known generally as perfluorocarbons and marketed by E. I. duPont under the name Nafion.
These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers containing an SO.sub.2 F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising one to eight carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbom atom adjacent the --SO.sub.2 F, particularly where the functional group exists as the --(--SO.sub.2 NH)mQ form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups.
These perfluorocarbons, while demonstrating significant resistance to most conventional corrosives, and substantial resistance to swelling in the presence of most conventional organic compounds, are none-the-less hydrophilic following hydrolysis. Typically these perfluorocarbons are used as cationic exchange materials and are therefore relatively readily permeable to cations. Where a substrate vulnerable to cation related corrosive attack is to be protected, this cation permeability and hydrophilic nature become undesirable.
Cation permeability of NAFION and hydrophilic properties are preponderately related to the functional group SO.sub.2 F and/or its derivatives. Typical derivatives include SO.sub.2 Cl, SO.sub.3.sup.- Z.sup.+ where Z.sup.+ is any alkali or alkaline metal cation, a quaternary ammonium ion or salt and hydrogen and SO.sub.2 O-ester. Were the functional groups to be modified to reduce the hydrophilic nature of the NAFION, a chemically resistant, highly nonporous coating and lining material would result.